The present invention relates to extruders in general, and more particularly to improvements in apparatus for extrusion of tubular parisons. Still more particularly, the invention relates to improvements in extruder heads for the making of tubular parisons which can constitute pipes or analogous hollow articles of constant diameter or which can be introduced into blow molding machines for conversion into bottles or other types of containers.
Italian Pat. No. 877,978 discloses an extruder head wherein a disk-shaped holder for the core in the die comprises an inner ring serving to support the core, an outer ring which is secured to the housing, and an intermediate ring which is disposed between the inner and outer rings and is connected thereto by radially extending webs. The plasticized material which flows toward the holder is divided into two tubular streams one of which flows between the inner and intermediate rings and the other of which flows between the outer and intermediate rings. The aforementioned webs divide each of these streams into several panels or fields. The two streams are supposed to merge downstream of the holder to form a single stream wherein the panels are in intimate contact with each other. The webs in the space between the inner end intermediate rings can be staggered with respect to the webs between the intermediate and outer rings (as considered in the circumferential direction of the core) in order to insure that the seams between the panels of the inner stream are angularly offset with respect to the seams of the outer stream. A drawback of the just described extruder head is that the full pressure which the plasticized material exerts against the core is transmitted to the housing by way of the holder. If the extruder head is designed for extrusion of large-diameter parisons, the aforementioned pressure can reach a value which is much too high for economical production of extruder heads. Moreover, such constructions are impractical in extruder heads wherein the annular chamber for the flow of plasticized material toward the orifice is formed with a storage space and the core must be moved axially of the housing in order to regulate the wall thickness of the parison, i.e., wherein the aforementioned holder must be mounted for movement toward and away from the extrusion orifice.
Another extruder head for subdivision of a first stream of plasticized material into two discrete streams which are thereupon caused to merge into a single stream is disclosed in BASF Schriftenreihe "Kunststoffverarbeitung im Gespraech" (3. Blasformen, 1973, pages 97-98). The mandrel which carries the core for the extrusion die has two heart-shaped cams and includes a sleeve which is located downstream of the cams. The upper end of the sleeve receives plasticized material by way of a channel which is located at the level of the inlet for admission of plasticized material. The material which issues from the plasticizing cylinder is caused to form two discrete streams as soon as it enters the housing of the extruder head, and each such stream flows along an arc of 90 degrees, as considered in the circumferential direction of the mandrel. The two streams are located diametrically opposite each other and one of these streams thereupon flows over and around the sleeve to form a first tubular body. The other stream flows into the sleeve and forms therein a second tubular body which is surrounded by the first tubular body. In other words, the material which is admitted into the housing of the just outlined extruder head forms two discrete tubular bodies without preceding formation of a single tubular stream. This results in a very pronounced resistance to the flow of plasticized material into and through the zones where the formation of two discrete tubular bodies occurs. Moreover, the surfaces along which the plasticized material flows are rather long; this, combined with very pronounced resistance to the flow of material toward the extrusion orifice, adversely influences the orientation of molecules in the plasticized material, i.e., the plasticized mass can develop internal stresses which do not disappear before the material leaves the extruder head. If such material is used for the production of blow molded articles, the internal stresses result in undesirable and unpredictable fluctuations of wall thickness of the ultimate product.
Still another drawback of the just outlined extruder head is that the mandrel must be provided with two heart cams one of which is defined by a portion of the mandrel having a first diameter and the other of which is defined by a mandrel portion having a different second diameter. This prevents the inflow of plasticized material in the form of mirror symmetrical streams. Moreover, a heart cam cannot insure the formation of a tubular stream of plasticized material whose wall thickness is constant all the way around the circumference of the mandrel. Thus, by using two discrete heart cams one of which forms an inner tubular stream and the other of which forms an outer tubular stream, a relatively thick portion of the outer stream is likely to overlie a relatively thick portion of the inner stream which is highly undesirable for obvious reasons.